5th Swiss Geoscience Meeting, Geneva 2007
Late Triassic - Jurassic Radiolaria, ultramafic and mafic
rocks define the Mesquito Oceanic Terranes – The
Nicaragua Basement
Baumgartner Peter Oliver, Flores Kennet & Bandini Alexandre Nicolas
Institut de Géologie et de Paléontologie, Université de Lausanne, Anthropole, 1015
Lausanne (Peter.Baumgartner@unil.ch, Kennet.FloresReyes@unil.ch,
Alexandre.Bandini@unil.ch)
The new Mesquito Oceanic Terranes (MOT) comprise the southern half of the
Chortis Block that has been assumed to be a continental fragment of NAmerica. The MOT are defined by 4 corner localities (Fig) characterized by
ultramafic and mafic oceanic rocks and radiolarites of Late Triassic-Jurassic
age: 1. Santa Elena, 2. El Castillo, 3 DSDP Legs 67/84, 4. Siuna.
The Santa Elena Ultramafic Unit (1) of N-Costa Rica together with the
serpentinite outcrops near El Castillo (2) in Southern Nicaragua are the
southernmost outcrops of the MOT. In the Las Brenes Quarry, 10 km N of El
Castillo (2) we discovered a diverse Rhaetian (latest Triassic) radiolarian
assemblage in a block tectonically embedded in serpentinite.
The Santa Elena Unit (1) itself is still undated, but it is thrust onto the middle
Cretaceous Santa Rosa Accretionary Complex that contains Early to Late
Jurassic radiolarite blocks, probably reworked from the MOT.
Serpentinites, metagabbros and basalts have long been known from DSDP Leg
67/84 (3), drilled off Guatemala in the Nicaragua-Guatemala forearc basement.
They have been restudied and reveal Ar/Ar-dated Late Triassic to mid
Cretaceous enriched Ocean Island Basalts and Early Cretaceous depleted
Island arc rocks of supposed Pacific origin.
We studied the Suina (4) area in NW-Nicaragua and defined the Siuna
Serpentinite Mélange, that contains, among high pressure metamorphic mafics
also Middle Jurassic (Bajocian-Bathonian) radiolarites in original sedimentary
contact with metabasalts. The Siuna Mélange also contains Late Jurassic black
detrital chert formed in a marginal basin shortly before subduction. The area
between localities 1-4 is largely covered by Tertiary to Recent arcs, but we
suspect that its basement is made of oceanic assemblages. Earth quake
seismic studies indicate an ill-defined, shallow Moho in this area. The MOT
covers most of Nicaragua and could extend to Guatemala to the W and form the
Lower (southern) Nicaragua Rise to the NE
The Chortis Block s. str., itself composed of several terranes with continental
records, is restricted to northernmost Nicaragua and Honduras (Fig).
South of The MOT, most of Costa Rica and Panama has basements that can
be assigned to the Caribbean Large Igneous Province (CLIP), a composite
plateau of eastern Pacific origin. Basement ages are rather homogeneous
throughout the area and range from Late Cretaceous (Coniacian) to Paleocene.
The Nicoya Complex In N-Costa Rica is an exception: Ar/Ar-dates on basalts
and intrusives indicate Early to Late Cretaceous ages. Middle Jurassic to Late
Cretcacous radiolarites occur as blocks (xenoliths ?) within the intriusives and
basalts. Our interpretation is that Jurassic, and perhaps early Cretaeous
5th Swiss Geoscience Meeting, Geneva 2007
radiolarite blocks were first accreted and formed part of the MOT, then became
reworked into the Nicoya Complex. This implies that the Nicoya Complex
formed at the edge of part of the MOT during the (mostly Late) Cretaceous.
Figure. New Terrane map of Southern Central America with the general plate
tectonic setting of the Caribbean Plate in the upper right. The Chrotis Block s.
str., characterized by Paleozoic continental basement, is restricted to NNicaragua, Honduras and E-Guatemala. Most of the basement of Nicaragua
and northernmost Costa Rica is supposed to be made of the newly defined
Mesquito Oceanic Terranes, sampled by DSDP Legs 67 and 84 and cropping
out in Siuna, El Castillo and Santa Elena. The Nicaragua Rise shows a twofold
bathymetry that is supposed to represent a twofold basement: The upper Rise
is continental to the N, the lower and irregular Rise is oceanic to the S. Oceanic
terranes of the Chorotega block (Costa Rica – Panama) are part of the
Caribbean Large Igneous Province (CLIP), a composite of plateaus. The Nicoya
5th Swiss Geoscience Meeting, Geneva 2007
Complex s. str. in the NW-Nicoya Peninsula exposes CLIP formed in contact
with the Mesquito Oceanic Terranes.